Microcontrollers see use in many instrumentation and control applications. In some applications, the microcontroller integrated circuits must be capable of both failsafe operation as well as low power operation. An example of an application involving failsafe and low power operation is a battery-powered detecting circuit for poison gas, such as carbon monoxide. Other examples of failsafe, low power applications include battery-powered medical devices that are implanted into the human body. Failsafe operation of such microcontroller integrated circuits allows the microcontroller to continue to execute instructions even after the failure of an external oscillator that supplies a clock signal to the microcontroller. If such a failure occurs, then a second clock signal is supplied to the microcontroller so that the microcontroller can continue to operate.
Circuits for automatically detecting a failure of a clock source and for switching a system clock from the failed clock source to a second clock source are known. See, for example, U.S. Pat. No. 6,341,355. If a failsafe device incorporating a microcontroller were to be provided with such an automatic clock switchover circuit, then the failsafe device would include two sources of clock signals. When a clock signal provided from one source fails, then the automatic clock switchover circuit would supply the microcontroller with a clock signal from the second clock source. Presumably both clock sources would be running at the same time so that when one of the clock sources would fail, the other clock source would be able to provide the clock signal needed to clock the microcontroller. Although failsafe operation is achieved, the power consumption of the overall device would likely be high because the overall device would include two clock sources that are consuming power.
In low power microcontroller applications, the power consumed by the oscillator that supplies the microcontroller with its clock signal may be a significant proportion of the total power consumed by the overall device. Battery lifetime may therefore be substantially adversely affected by the need to provide power to two clock sources. A solution is desired whereby failsafe operation can be provided, while at the same time reducing power consumption relative to conventional automatic clock switchover circuits.